I bought a spark fun inventors kit in order to learn about arduino. I am attempting to complete circuit 13 http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Kits/SFE03-0012-SIK.Guide-300dpi-01.pdf . It is a circuit to control 2 LEDs using a relay. I know I have the relay hooked up correctly because I can hear it click, but instead of the first LED turning off and the second LED turning on the first LED dims. Then when the relay clicks again the first LED brightens. I'm not sure what I have done wrong but no matter how many times I undo the circuit and start over I get the same result. I am especially confused because from what I understand a relay functions like a digital on/off switch. So I cannot understand why the first LED would dim instead of just turning off, even if I didn't have the second LED connected correctly.

Any tips/pointer/advice would be really appreciated. This is driving me nuts. Thanks.

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    \$\begingroup\$ I assume "first relay" and "second relay" in your question should really be "first LED" and "second LED". It is possible that the pin arrangement of the relay is not the same as described in the book. Check the pinout of the relay with an ohmmeter to see if it matches the schematic. \$\endgroup\$ – Peter Bennett Dec 27 '13 at 5:41
  • \$\begingroup\$ @PeterBennett You are correct. I was thinking about relays so I typed that instead of LED. I have edited. I will take your advice and check with an ohmmeter. I just have to get a new battery since apparently I left it on last time I used it. \$\endgroup\$ – MichelleJS Dec 27 '13 at 6:00

It seems that:

  • when the LED dims, current is flowing through the coil and then through the LED. Because the coil allow low current to flow through it, you see low brightness into your LED.
  • when the relay clicks again an high voltage is produced on the coil and the LED, then you see an high brightness in your LED. This is interesting if you are a novice because you can see your LED doing the function that, in the original circuit, would be performed by flyback diode.


  • flip your relay left-right
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  • \$\begingroup\$ Sorry for the delay, I was busy with the holidays. So I have hooked everything back up again and am again getting the same results. I'm not sure what you mean by flip the relay left-right, since there are different pins on each side. \$\endgroup\$ – MichelleJS Jan 7 '14 at 21:55
  • \$\begingroup\$ Also you said the LED is performing the function that the flyback diode is supposed to do? Does this mean I have the flyback diode configured incorrectly? I also still don't understand why the second LED would not turn on? \$\endgroup\$ – MichelleJS Jan 7 '14 at 21:57
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    \$\begingroup\$ Ok your answer completely led me to the correct answer. Thankyou so much in the diagram the black band on the diode is not attached at the 5 volt end, which is what I was looking at. However in the comments about hooking up the circuit it says that the black band needs to be connected to 5 volts. Once I flipped the diode 90 degrees everything worked perfectly. Thats what I get for relying on the picture and not carefully reading the instructions. \$\endgroup\$ – MichelleJS Jan 7 '14 at 22:03

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